1. Field of the Invention
The present invention relates to a biodegradable plastic composition, a molded article thereof and a method for controlling a biodegradation rate using the same composition. In more detail, the present invention relates to a biodegradable plastic composition which exhibits greatly improved resistance to hydrolysis and weather, in particular the former, by compounding specified agents therein and has excellent durability resulting from the improved resistance. Applications of the composition include a biodegradable molded article. A method for controlling a biodegradation rate of the biodegradable plastic is also disclosed.
2. Description of the Prior Art
Recently, research and development of biodegradable plastic have been promoted because of increasing attention to a need for and a significance of environmental preservation and problems such as environmental contamination caused by plastic waste.
The biodegradable plastics are roughly classified into aliphatic polyesters having polyester groups in a molecular backbone thereof (including those produced by microorganisms), natural macromolecule type polymers such as cellulose, polysaccharides produced by microorganisms, polyvinyl alcohol (PVA) and polyethers such as polyethylene glycol (PEG).
Among them, aliphatic polyesters have not been utilized as the biodegradable plastic because they have, in general, low melting points and poor heat stabilities in their production stages, and a sufficiently large molecular weight has not been obtained to provide physical properties suitable for practical molded articles. However, with technological developments to solve these problems, a high molecular weight of aliphatic polyester has come out and started to be used as materials for agriculture, forestry and fishery (mulch films, plantation pots, fishing lines, fishing nets, and the like), materials for civil engineering work (water holding sheets, nets for plants, sandbags, and the like) and packaging and containers (those difficult to recycle due to adhered soil, foods, and the like).
The biodegradable plastics including the above-described aliphatic type polyesters, however, should not only have the same level of functions as the conventional plastics (in characteristics such as strength, water resistance, formability and heat resistance) in use, but also be rapidly degradable by microorganisms generally present in nature.
Based on these situations, several methods for controlling the degradation rate of the biodegradable plastic have been proposed. For example, a method aiming at shortening of a degradation period by an addition of hydrolase (disclosed by JP-A-4-168149 (refer to claims or the like)), and another method aiming at an extension of the degradation period by reducing unreacted monomers and impurities in a polymer and low molecular weight compounds such as linear or cyclic oligomers (disclosed by JP-A-9-12688 (refer to claims or the like)), are known.
Notwithstanding these proposals, however, the biodegradable plastic consisting of the aliphatic polyester according to the conventional art has such problems as a hydrolysis reaction progressing in the aliphatic polyester induced by exposure to moisture in an ambient atmosphere or by being heated during a production process of pellets-which are raw materials of plastic products, or in a production process of products from said pellets, leading to lowering or dispersion of initial properties of molded articles as well as an unstable biodegradability of products. Therefore, adjustment of the biodegradability, that is, biodegradation rate, is not yet sufficient.
The present inventors proposed, as disclosed by, e.g., JP-A-11-80522 (refer to claims or the like), a biodegradable plastic composition with an adjusted biodegradation rate by compounding a carbodiimide compound into the biodegradable plastic. This proposal improved a hydrolysis resistance or a function for adjusting biodegradability, although not to a sufficient extent. The proposal also had another problem of losing transparency of the product, in particular, in a polylactic acid based biodegradable plastic product with an increased compounding amount of the carbodiimide compound to enhance the hydrolysis resistance.
JP-A-2001-525473 (refer to claims or the like), for example, also proposes a method aiming at an extension of the degradation period by compounding an aromatic carbodiimide as a hydrolysis stabilizer to the biodegradable polymer. This method, however, involves problems, because the biodegradable polymer it gives shows insufficient, although improved, hydrolysis resistance, decreased transparency irrespective of an additional amount of the stabilizer, and insufficient weather resistance.
Moreover, JP-A-6-184417 (refer to claims or the like), for example, proposes a lactic-based composition characterized by a lactic-based polymer as a biodegradable polymer compounded with at least one type of additive selected from the group consisting of ultraviolet absorber and light stabilizer at 0.001 to 5 parts by weight per 100 parts by weight of the polymer. However, this invention is aimed at controlling decomposition of the polymer by solar rays, but not at controlling hydrolysis.
Still more, JP-A-2002-114893 (refer to claims or the like), for example, proposes a thermoplastic polyester resin composition, although not biodegradable, characterized by a thermoplastic polyester resin compounded with a resin-based ultraviolet absorber and aliphatic-based polycarbodiimide compound to improve resistances of the resin, which has relatively high heat resistance, to weather, alkali and hydrolysis. However, the resin-based ultraviolet absorber disclosed by the document involves a problem of being not compatible with a biodegradable plastic.